Settings View Source Membrane.WAV.Parser (Membrane WAV Plugin v0.8.0)

Element responsible for parsing WAV files.

It requires WAV file in uncompressed, PCM format on the input (otherwise error is raised) and provides raw audio on the output. WAV header is parsed to extract metadata of the raw audio format. Then it is dropped and only samples are sent to the next element.

The element has one option - frames_per_buffer. User can specify number of frames sent in one buffer when demand unit on the output is :buffers. One frame contains bits per sample * number of channels bits.

wav-header

WAV Header

   0                   4                   8                   12                  16
   _________________________________________________________________________________
0  |                   |                   |                   |                   |
   |      "RIFF"       |    file length    |      "WAVE"       |       "fmt "      |
   |                   |                   |                   |                   |
   |___________________|___________________|___________________|___________________|
16 |                   |         |         |                   |                   |
   |   format chunk    | format  |number of|      sample       | data transmission |
   |      length       |(1 - PCM)|channels |       rate        |       rate        |
   |___________________|_________|_________|___________________|___________________|
32 |  block  |  bits   |                   |                   |                   |
   |  align  |  per    |      "fact"       |     fact chunk    |    samples per    |
   |  unit   | sample  |                   |       length      |      channel      |
   |_________|_________|___________________|___________________|___________________|
48 |                   |                   |                                       |
   |      "data"       |    data length    |                 DATA                  |
   |                   |     in bytes      |                                       |
   |___________________|___________________|_______________________________________|

Header may contain additional bytes between bits per sample and "fact" in case of format different from 1 (1 represents PCM / uncompressed format). Length of block from format until "fact" is present in format chunk length (it is 16 for PCM).

Blocks from byte 36 to 48 are optional. There can be additional bytes after samples per channel if fact chunk length contains number bigger than 4.

parsing

Parsing

Stages of parsing:

• :init - Parser waits for the first 22 bytes. After getting them, it parses these bytes to ensure that it is a WAV file. Parser knows format chunk length and format, so it is able to raise an error in case of different format than 1 (PCM) or different length than 16 (for PCM). After parsing, the stage is set to :format.
• :format - Parser waits for the next 22 bytes - fmt chunk (bytes 20 - 35) without format and either "fact" and fact chunk length or "data" and data length in bytes. Then it parses it and create Membrane.RawAudio struct with audio format to send it as stream_format to the next element. Stage is set to :fact or :data depending on last 8 bytes.
• :fact - Parser waits for 8 + fact chunk length bytes. It parses them only to check if the header is correct, but does not use that data in any way. After parsing, the stage is set to :data.
• :data - header is already fully parsed. All new data from the input is sent to the output.

pads

Pads

input

:input

output

:output